The present invention generally relates to active devices, and more particularly to a method of fabricating an active device having a high temperature superconductor layer on an insulating substrate.
In the integrated circuits, a long interconnection strip is used for connecting various devices formed on the integrated circuit. With the increase in the integration density, the length of the interconnection strip is increasing more and more. For example, there is an integrated circuit in which the length of the interconnection reaches as much as 2 cm. As will be easily understood, such a long interconnection strip causes a considerable delay in the signals that are transferred therethrough, and the speed of logic gates is now becoming limited by the delay caused by the interconnection strips rather than the operational speed of the logic device itself. This is particularly conspicuous in the integrated circuits of high speed logic devices such as Josephson devices.
The delay in the signal transfer in the interconnection strip is mainly determined by the parasitic capacitance and the resistance of the strip. Thus, a material that has the low resistance is preferable for the interconnection strip. Conventionally, polysilicon, silicides, tungsten, molybdenum, aluminum, and the like, are used for the interconnection strip, wherein aluminum provides the lowest resistance and is used widely. Even so, the resistance of the interconnection increases with increasing integration density, as such an increase in the integration density inevitably results in the reduced thickness of the aluminum or other conductor strip.
Meanwhile, the discovery of ceramic superconductors that show a relatively high critical temperature has opened a new field in the research of active devices and integrated circuits. With the use of these so-called high temperature superconductors for the interconnection strips, it is expected that one can achieve the zero resistance relatively easily, without cooling the entire device to the liquid helium temperature. There is even an expectation for the discovery of material that causes the transition to the superconducting state at the room temperature. Thereby, the problem of delay in the signal transfer caused by the interconnection strip would be eliminated. Further, such a superconducting interconnection strip of high temperature superconductors would be advantageous for constructing Josephson integrated circuits.
In the integrated circuits, patterned interconnection strips are provided on an oxide film that is formed on the surface of a semiconductor substrate. The oxide film thus formed is generally in the amorphous state, and because of this, there occurs a problem when one deposits a layer of ceramic or oxide-based superconductors on such an amorphous layer. The superconductor layer cannot be formed on the amorphous film as long as conventional sputtering, evaporation-deposition, CVD, or MBE processes are employed. Thus, the material on which the high temperature superconductors can be deposited is generally limited to the single crystals of MgO, SrTiO.sub.3, YSZ, Si, or the like. These materials allow the epitaxial growth of the high temperature superconductor layer deposited thereon.
When using the high temperature superconductors in the integrated circuits, however, one encounters another problem that the superconductor layer has to be grown directly on the amorphous, insulating oxide layer. Because of this problem, use of high temperature superconductors for the interconnection strips has been hitherto unsuccessful in the active devices and integrated circuits.